docs: def get_balance_on(self, on_date: datetime) -> Decimal:
          """ Returns the balance on (and including) a certain date """
          assert isinstance(on_date, datetime)
  
          total = Decimal(0)
  
          splits = self.get_splits_up_to(on_date)
  
          for split in splits:
              total += split.quantity * self.account.sign
          return total